1. Field of the Invention
The present invention concerns a vertical-cavity surface-emitting laser diode (hereinafter referred to as VCSEL) and, it specifically relates to a protective structure for protecting the VCSEL against electrostatic damage.
2. Description of the Related Art
The VCSEL has excellent features in that it has low threshold current, small power consumption and can obtain a circular light spot easily, and light sources can be arranged as a two-dimensional array, and it has been expected for use as a light source in optical communication equipment or electronic equipment.
Like other semiconductor devices, the VCSEL is sometimes exposed to a high voltage by static electricity in handling of circuit substrates, etc. If electrostatic discharge (hereinafter referred to as ESD) should occur inside the device, since a large spike current flows instantaneously, this destroys or deteriorates the device causing a failure not capable of conducting normal operation. Several reports have been made for coping with such problems.
In Japanese Published Unexamined Patent Application No. Hei 11-112026, a protective device is provided separately from a light emitting device considering that the withstand voltage is as low as 50 V in the reverse direction and 150 V in the forward-direction in semiconductor light emitting devices, particularly, gallium nitride compound semiconductors. As the protective device, a Zener diode or a transistor is used, for example, thereby short-circuiting the reverse voltage or a forward voltage higher than the working voltage possibly applied to the light emitting device.
In U.S. Pat. No. 6,185,240B1, a diode as a countermeasure for ESD and VCSEL are integrated to form both of them on one identical chip. A trench is formed on the substrate to define a diode region. The diode is, preferably, a p-i-n diode having a breakdown voltage of from 10 to 20 V.
Further, Bobby M. Hawkings et al., “Reliability of Various Size Oxide Aperture VCSELs” Honeywell, 2000 is a study report on the reliability of a selective oxidation type VCSEL and describes a relation between the breakdown voltage due to ESD and oxidized aperture. In this report, ESD damage is tested by a human body model according to US MIL standards, and an oxidized aperture size of from 5 to 20 μm is used as a sample. When a pulse voltage in the forward direction or the reverse direction is applied to the VCSEL and the optical output power changes by −2 dB, it is defined as damage or failure. FIG. 9 of the reference shows the result of the ESD damage test. According to the result, it is considered that ESD damage is a function of an oxidized aperture diameter or area and the ESD breakdown voltage increases as the oxidized aperture diameter increases.
However, the existent protective device for the VCSEL has the following problems. Since the protective device is disposed separately from the light emitting device in Japanese Published Unexamined Patent Application No. Hei 11-112026, in a case of handling the light emitting device as a single component, the ESD countermeasure is still insufficient. Further, this increases the number of elements constituting the laser device to increase the cost.
Integration of VCSEL and the protective diode on one chip in U.S. Pat. No. 6,185,240B1 may be a countermeasure for ESD during handling, but plural trenches have to be formed when the protective diode is formed on the substrate, which complicates the steps and cannot always form the diode easily.
While Bobby M. Hawkings, et al., “Reliability of Various Size Oxide Aperture VCSELs” Honeywell, 2000 shows that the ESD withstand voltage increases in proportion with the oxidized aperture diameter, desired basic laser characteristics cannot be obtained by merely increasing the oxidized aperture diameter. Particularly, in a single mode VCSEL, the oxidized aperture diameter tends to be decreased, which inevitably lowers the ESD withstand voltage.